1. Field of the Invention
The present invention relates to electrooptic display devices, and more specifically to liquid-crystal panels, used in transmission or in reflection, or even in projection onto a screen.
2. Discussion of the Background
Most liquid-crystal panels suffer from a major drawback, this being the limited viewing angle at which they can be observed: as soon as the viewing angle departs from the normal to the surface of the panel, the contrast between white and black decreases considerably and degrades the image presented.
The object of the present invention is to provide novel technical solutions allowing the panel to be viewed with a satisfactory contrast at a viewing angle sufficiently away from the normal.
Hereinafter, the panels of interest will be mainly those in which the liquid crystal is of the twisted-nematic type and is placed between two crossed polarizers, so that the light is transmitted when there is no excitation of the crystal by an electric field and, on the other hand, is interrupted when an electric field is present. However, the invention could be used in other types of panels, and especially in a configuration having parallel polarizers where the light is interrupted when there is no electrical excitation.
The cause of the contrast deficiency at non-zero incidence will be discussed later, but for the moment it may be stated that this contrast deficiency arises from the natural birefringence of the material of which the liquid crystal is composed. The actual principle of operation of the liquid crystal relies on the dielectric anisotropy of the molecules and on the birefringence which results therefrom. The dielectric anisotropy allows the molecules to be oriented by an electric field; the birefringence modifies the polarization of the light. The combined action of these properties is very efficient in the case of light rays which pass through the panel perpendicular to the surface, but it is much less efficient in the case of rays at oblique incidence. This generally results in much poorer contrast as soon as the viewer views a cell of the panel at a viewing angle not perpendicular to the panel.
In order to try to remedy this deficiency, it has already been proposed in the prior art to combine the basic structure of the liquid-crystal panel with structures which tend to compensate for the variations in birefringence as a function of the angle of incidence.
These structures rely
on an analysis of the birefringent action of the liquid crystal in the excited state (in the presence of an electric field), PA1 on an estimation of the phase retardations introduced between the two polarization components of the light by the birefringence of the liquid crystal as a function of the viewing angle and PA1 on the insertion of compensation films which introduce a phase difference, if possible the reverse of the estimated phase difference in the liquid crystal.
Various solutions have been described, which are more or less satisfactory, but the object of the present invention is to propose a novel technical solution to this problem.
In the technically most advanced solutions, it is proposed to use a compensation film made of a molecular material which is optically a negative uniaxial medium (the definition of such a medium will be given later), since overall it may be estimated that the liquid crystal behaves as a positive uniaxial medium, and consequently compensation using a negative uniaxial medium is a priori the most appropriate solution.
Patent EP-A-0,576,342 proposes a negative uniaxial compensation medium having an axis which is inclined with respect to the plane of the panel.
Patent EP-A-0646,829 proposes the use of two inclined negative uniaxial media.
Patent EP-A-350,383 also proposes one or two negative birefringent films.
Patent EP-A-349,900 proposes compensation using two positive uniaxial films of axes lying in the plane of the panel.
In all these solutions, the difficulty is to physically produce compensation films which actually have the desired birefringence properties. The known techniques are techniques of deposition, of stretching of films and of polymerization in an electric field or under ultraviolet illumination.
In the case of compensation using a film having an optical axis perpendicular to the plane of the film, it has been envisaged to produce the birefringence by superposing thin transparent layers of alternating optical index; U.S. Pat. No. 5,196,953 describes such a solution. Similarly, Jinn-Chou Yoo and Hang-Ping D. Shieh have proposed to produce virtually this structure of plane layers of alternating indices in the form of a volume hologram which reconstitutes index variations identical to those in the superposition of plane layers. This is explained in the article entitled "Novel Compensator with grating structure for twisted nematic liquid crystal display applications" coming from "Conference record on the 1994 International Display Research Conference and International Workshop on Active-Matrix LCD's and Display Materials, Monterey".